Recombination and trapping processes in a semiconductor
Recombination is the process where an electron moves from the conduction band into the valence band so that a mobile electron-hole pair disappear. Classical mechanics requires that momentum be conserved in an encounter of two particles. Since the momentum is zero after recombination, this conservation law requires that the “colliding” electron and hole must have equal magnitudes of momentum and they must be travelling in opposite direction. This requirement is very stringent, and hence the probability of recombination by such a direct encounter is very small.
The most important mechanism in silicon or germanium through which holes and electrons recombine is that involving traps, or recombination centers, which contribute electronic states in the energy gap of the semiconductor. Such a location acts effectively as a third body which can satisfy the conservation-of-momentum requirement. These new states are associated with imperfection in the crystal. Specifically, metallic impurities in the semiconductor are capable of introducing energy states in the forbidden gap. Recombination is affected not only by volume impurities but also by surface imperfections in the crystal.
Gold is extensively used as a recombination agent by semiconductor device manufacturers. Thus the device designer can obtain desired carrier lifetimes by introducing gold into silicon under controlled conditions. Carrier lifetimes range from nanoseconds( 1 ns = 10-9 sec) to hundreds of microseconds (µsec).